It is known that the sedimentary portion of the earth's surface is made up of successive layers or beds which generally do not have a constant thickness and exhibit a certain dip, e.g., an inclination with respect to a horizontal plane. This inclination may be defined as the angle formed by a line in the plane of the considered layer having the greatest slope in relation to a horizontal plane. It is understood that the dip of formation layers penetrated by a borehole constitutes extremely important information in petroleum prospecting. Such information is useful for evaluating the chances of obtaining hydrocarbons from a borehole, for establishing the nature of adjacent geological structures and for choosing the location of new boreholes.
Dip can be determined from resistivity logs which have lateral separation provided by using measurement electrodes placed on four pads carried by a dipmeter probe which are applied against the borehole wall at points corresponding to two perpendicular diameters. Typically, each pad contains a measurement electrode or transducer which conducts an investigation of formation characteristics immediately adjacent the pad. Thus, the investigations take place along four paths distributed at about 90.degree. intervals around the circumference of the borehole. When the probe is moved vertically through the borehole, four logs are obtained and recorded, usually in the form of sampled electrical signals which indicate by features such as peaks, dips and bumps, the presence of bedding planes along the respective vertical paths. The respective offset needed to bring corresponding signal features into depth alignment makes it possible to determine the dip of the formation in relation to the axis of the dipmeter probe. To determine the offset required between two signals, conventional correlation techniques or pattern recognition between possibly corresponding features on each signal may be employed.
In the prior art dipmeter probes used as described above, it is important that each of the pads maintain their expected position against the borehole wall. Since many boreholes are not circular in cross-section but often resemble elipses or off-center circles with different diameters, each pad is attached to the main probe body in a manner which will allow it to seek the desired position against the borehole wall as independently as possible. However, since dip information is desired, it is required to restrain the relationship between or independently measure the diameters between opposing pairs of pads. It is for this reason that four-pad dipmeters usually include the recording of two orthogonal diameter calipers. The diameter information from both calipers is required in order to determine dips corresponding to formation layers. The corresponding pad-to-pad distances between at least three pads located at approximately 90.degree. circumferential separations around the borehole is combined with offsets between signals obtained from the three pads.
Unfortunately, signals obtained from circumferential segments separated by 90.degree. or more (for the three-pad dipmeter, the separation corresponds to 120.degree.) are not as alike as one would assume for several reasons. First, the pad supporting a given measurement electrode may not maintain wall contact in a consistent manner. This is due to the fact that the wall of the borehole is not uniformly smooth, but exhibits roughness, bumps and cavities. It is also known that the rubbing of the pads on the borehole wall is not uniform along the borehole or from one pad to another in the same section of the borehole. Further, for boreholes deviated substantially from vertical, the weight of the tool tends to collapse the mechanical mechanism attempting to apply all the pads against the side of the hole to the extent that the pad lying on the topmost side of the hole will float away from the borehole wall. This results in substantial distortions, both of the signal obtained from the floating pad relative to other pads in proper position against the wall, and in the diameter appropriate for use with the signal. Thus, there are several non-formation characteristic related factors which tend to introduce noise or change the nature of electrical signals obtained from the different pads of the dip-meter probe.
One attempt to enhance dipmeter signals over noise is described in U.S. Pat. No. 3,521,154 which issued July 21, 1970 to J. J. Maricelli. As shown therein in FIGS. 1 and 2, two substantially identical electrodes are placed on the same pad, one above the other and separated by a fixed vertical distance d.sub.1. The signal from the uppermost electrode (leading when the probe is being withdrawn from the borehole) is memorized for a depth difference corresponding to d.sub.1 and multiplied with the signal obtained from the lower or trailing electrode to produce a new signal for the pad having a better signal-to-noise ratio. Since the two electrodes are aligned vertically and investigate the same slice of formation along the same path on the borehole wall, but do this same investigation at different times, this technique provides a new pad signal compensated for noise occurring at different random times.
In an embodiment disclosed in FIGS. 5A and 5B of the U.S. Pat. No. 3,521,154 and described at column 7 beginning at line 13, three substantially identical electrodes are placed on the same pad, two of which are vertically displaced and a third horizontally and vertically displaced relative to the first two. The signals from the leading electrodes are depth shifted so that all three signals representative of the same depth in the borehole are present simultaneously. Then as described in column 8, line 1, the three signals are multiplied together. Because of the vertical and now horizontal displacements between the three electrodes, any noise brought about by inconsistencies with time and vertical movement or by actual differences in the formation over the small horizontal displacement will be reduced in the resulting signal product. It is important to remember that the depth shifting between these common pad signals is fixed and corresponds to the fixed vertical distance between the electrodes and that the depth shifted signals are continuously multiplied together to form a single new signal, irrespective of the properties of the individual signals. Thus inconsistencies between the signals from the electrodes are considered as noise due to the borehole environment, such as localized caves, and thus cancelled out.
While it has long been known that sedimentary formations vary to considerable extent in the direction of deposition; i.e., in their vertical direction, and attempts have been made to measure such vertical anisotropy, it has been recently discovered that some formations are anisotropic in the horizontal direction as well. The true characteristics of many important petroleum bearing formations can be expected to vary from one point on the circumference on the borehole wall to another point at the same depth. The presence of such horizontal anisotropy, particularly for characteristics which control fluid flow, as for example permeability, must be considered when evaluating reservoir potentials, since these directional properties affect the way fluids enter into the borehole or may be injected from the borehole into the formation, as for example, during flooding operations.
In some cases, the horizontal anisotropy is characteristic of the formation deposition, as for example, in the case of foreset bedding or elongated sand grains which are preferentially oriented by air or water currents at the time of deposition. Other characteristics which change in the horizontal or lateral direction are introduced after deposition such as highly elongated pores developed by solution along stress lines in carbonates or inclined fractures resulting from flexural distortions in formations that are too brittle to elastically yield to these forces. Since the presence of these types of lateral changes in formations often has a profound effect upon the accumulation or producibility of petroleum deposits, a knowledge of the presence and if possible, the orientation of these changes in subsurface formations, is important.
Prior art devices which attempt to measure lateral characteristics of formations include apparatus designed to conduct investigations of formation characteristics in the lateral direction. Typically, a transmitting transducer will introduce a signal at one point on the circumference of the borehole which is subsequently received by a receiving transducer at another point on this circumference with the points typically separated by 90.degree. or more. In order to conduct investigations which propagate signals through each 90.degree. quadrant, both transmitting and receiving transducers are mounted on separate pad-like projections, with each pad typically containing one transmitting and one receiving transducer mounted one above the other. One apparatus of this type is disclosed in U.S. Pat. No. 3,794,976 issued on Feb. 26, 1974 to B. E. Mickler. In the embodiment described in this patent, acoustic type transducers are employed to conduct a circumferentially directed investigation of acoustic characteristics of the formation between four transmitter and receiving pairs spaced over 90.degree. quadrants. Signals obtained over one quadrant may be compared with the signals obtained over other quadrants to detect lateral changes in formation from quadrant to quadrant, such as intervening vertical fractures.
Additional apparatus have been described in the prior art for conducting measurements alternately along paths parallel to the vertical axis of the borehole and radially around the borehole. For example, U.S. Pat. No. 3,564,914 issuing Aug. 12, 1968 to K. P. Desai, et al. discloses a pad configuration for measuring both differential acoustic and resistivity measurements in the vertical and the radial directions. Of particular interest, in FIG. 6 thereof is disclosed a five-transducer arrangement allowing the vertical axis investigation embodied in pad 18 as shown in FIG. 2 to be conducted along two mutually perpendicular axes, e.g., in the vertical and radial directions along the borehole wall, as described at the bottom of column 4 bridging through column 5. It is important to note that housings 24 or 26 are capable of performing either as electrical or acoustic transducers. As electrical transducers, current is emitted from an electrode in housing 24, returning to the backplate 66 of pad 18 and the potential difference between electrodes mounted in two housings 26 produced as the measurement signal. In this manner, the electrical measurement investigates the difference in the measured characteristic between electrodes along a line common to measurement housings 26 and current housing 24. Similarly, in an acoustic investigation, transducers in housing 24 transmit energy which is received by transducers in housings 26.
These arrangements produce a single signal corresponding to the difference in signals between housings 26 as the signal propagates through the formation along a line between the housings. This line may be either fixed in a horizontal or vertical direction, or both as illustrated in FIG. 6. By nature of such differential measurements conducted along two points on a line also containing the signal source, it is inherent that a signal, whether acoustic or electrical or otherwise in nature, will be different between two measurement points because of signal gradients in that direction, since the signal is also introduced at another nearby point along that same line. These measurement signal differences will exist solely due to geometrical spreading and attenuation losses, for example, and would occur even if the formation were homogeneous. Thus, signals representative of differential measurements between identical transducers lying along lines corresponding to gradients in the signals do not readily lend themselves to indicating changes in characteristics between the transducers because the signals are inherently different.
It is an object of the present invention to produce signals which readily indicate changes in characteristics of the formation occurring between measurement transducers and which have no inherent differences under homogeneous formation conditions.
A deviation from the differential measurement, while retaining the attributes of a circumferential measurement, is suggested in U.S. Pat. No. 2,963,641 issued Dec. 6, 1960 to R. H. Nanz. In addition to providing a description of some types of lateral characteristics of earth formations of interest in the present application, this patent illustrates in FIG. 2 a short spacing acoustic measurement between one transmitter 21 and one receiver 22 mounted on each of four pads in a side-by-side relationship. However, since only one transducer is a measurement transducer, only one signal is produced per pad and it is necessary to compare signals produced from different pads separated by 90.degree. or more of circumference to determine the presence of and/or direction of any lateral change. The ability to fix the distance corresponding to the investigation to that between the side-by-side transducers overcomes the uncertainty in path length present in other circumferential measurements conducted from one pad to another, but however, still includes the uncertainties inherent to differences in the manner different pads are applied to the borehole wall. This problem, inherent in pad-to-pad signal comparisons, limits the ability to distinguish changes corresponding to measurement conditions from pad to pad from changes in the lateral formation characteristics.
It is an additional object of the present invention to provide signals for investigating lateral changes in formation characteristics which are essentially free of effects of changes in circumferential pad-to-pad measurement conditions.
Another problem present in producing signals for comparison is illustrated in the application of a so-called "speed electrode" employed with more sophisticated dipmeter probes. In this application, two identical electrodes similar to those shown in the aforementioned U.S. Pat. No. 3,521,154 are displaced one above the other on a common pad. However, in this case the two signals produced from each electrode are not combined to produce a single signal with a superior signal-to-noise ratio but are correlated one with another to determine an apparent displacement which should compare with the actual displacement between the two electrodes. A difference in these displacements indicates a variation in the speed of the pad during the period between when the speed electrode and the subsequent measure electrode pass the same feature on a borehole wall.
These variations in speed are common to pad type probes and appear to represent a sort of skipping or loading and unloading of the friction between the pads and the borehole wall. The speed variation is commonly known as "yo-yo" and introduces an uncertainty both in the actual signal and in the sampling of signals acquired at different vertical points along the borehole, since such sampling is usually determined at intervals either related to the cable motion at the surface or small increments of time, but in either case not directly related to the actual motion of the pad along the borehole. Thus, signals produced at different times, corresponding to different points along the borehole in the direction of movement, introduce when compared, an uncertainty as to whether the differences observed between these signals is related to an actual change in the formation characteristics or to a change induced because of the sampling of the signals at two different times.
Accordingly, it is a further object of the present invention to produce signals which may be sampled at substantially the same times and same position of the probe as it moves along the borehole. This provides the advantage that such signals will be free of time varying sampling uncertainties.
The conventional dipmeter probe provides four substantially identical measurement electrodes on different pads. The linkage employed with the more sophisticated of these probes provides that electrodes on each of these pads remain substantially in the same plane and that this plane remains normal to the axis of the dipmeter probe which is generally parallel if not coincident to the axis of the borehole.
It has been previously recognized that it would be advantageous to have more than the four circumferentially spaced signals now available for dipmeter correlation. When the lateral spacing between signal transducers becomes large, as is the case for the four pad probe in large radius boreholes, even where formations vary only slightly in lateral characteristics, it becomes difficult to correlate with certainty a feature clearly present on one signal to a possibly corresponding feature present on a signal from a pad located 90 degrees or more around the borehole. A similar problem occurs when bedding planes intersect the borehole at high angles (due either to high angle dip or boreholes highly deviated from vertical) because one pad may be looking at the bedding planes at one angle while the next pad looks at these planes at a substantially different angle, as for example, one along the strike and the other across the bedding planes. Here the variation in bedding plane intersection angles over 90 degrees leads to substantial differences in signal features for the same bedding plane. If an intermediately spaced signal could be provided, the lateral variation could be expected to be less over the smaller spacing and the transition of a given feature much easier traced from one signal to the next around the borehole.
However, providing such desirable intermediate signals, i.e., signals closer than 90 degrees of circumference or from measurement transducers spaced along the borehole wall at distances less than the minimum radius of the borehole (a circular borehole wall is 2 .pi. or 6 plus radii in circumference) is not as easy as it might appear. The size of the mechanical linkage and pad supports limits the number of pads that can be placed in a single common-plane array and still operate in a small diameter borehole. Tandem arrays of three or four pads with each array in different planes spaced vertically one above the other and rotationally offset from each other, might appear to be a way of providing signals at circumferential separations less than the radius of the borehole. However, when the effect of "yo-yo" motion and the resulting sampling distortions discussed above for vertically spaced electrodes are considered, particularly for the substantially larger vertical spacings necessary for tandem arrays; along with uncertain rotation rates making the rotational offset variable (rotation could be such that pads in one tandem array track, rather than offset, pads in the other array), the tandem pad array approach does not solve the problem of producing simultaneous signals from more than four fixed spacing points around the borehole circumference.
It is therefore a still further object of the present invention to provide signals from measurement transducers located substantially in the same plane and spaced around the borehole at fixed spacings less than the minimum borehole radius.
It has been apparent from optical records of prior art dipmeter signals that substantial differences exist in the nature of signals obtained from different quadrants of the borehole. Signals obtained from opposing pads (spaced at 180.degree.) often resemble one another more than signals obtained from adjacent pads (typically separated by 90.degree.) thus, providing an indication that the formation characteristics in one direction appear to be different from the characteristics in another direction. Thus, if not due to pad measurement problems, these differences would indicate that lateral anisotropy must be present. However, there has been no successful technique for directly indicating the degree of lateral anisotropy or horizontal homogeneity.
For example, recent use of four-pad dipmeter signals optically recorded in a special presentation adapted for comparing signals from adjacent quadrants has facilitated the identification of vertical fractures. This technique and presentation is described in an article entitled, "Reservoir Evaluation of Fractured Cretaceous Carbonates in South Texas", by J. Beck, A. Schultz and D. Fitzgerald, published in the SPWLA 18th Annual Logging Symposium, June 5--8, 1977 TRANSACTIONS. While this special presentation is helpful, it does not provide direct assessment of the degree of lateral homogeneity of formation conditions in a particular direction. For example, significant differences between signals from two adjacent pads may exist when a substantially vertical fracture appears in front of one of the pads reducing the resistivity signal obtained from that pad. This technique generally indicates lateral change only to the extent that the fractures present immediately adjacent one pad do not exist immediately adjacent other pads providing signals in the other quadrants of the borehole. It would be advantageous to have a technique which automatically provided the lateral or horizontal homogeneity or lateral changes of a local point immediately adjacent each pad, rather than between pads scattered around the borehole at distances which vary with the borehole diameter.
Accordingly, another object of the present invention is to continuously provide an automatic indication of the horizontal or lateral changes in characteristics of an earth formation at local points around the borehole penetrating the formation.
It is a still another object of the present invention to produce signals indicative of lateral characteristics of an earth formation without reliance on the similarity of measurement conditions from one pad to another or variations in distance between adjacent or opposed pads which would affect the determination of changes in lateral characteristics of the formation between these pads.
In accordance with the invention, method and apparatus for investigating lateral characteristics of an earth formation penetrated by a borehole are provided. The invention comprises producing first and second signals respectively from substantially identical first and second measurement transducers mounted on the same pad adapted for application against the borehole wall. These transducers are mounted alongside each other in a plane normal to the direction of movement along the borehole and at a fixed center-to-center separation which is less than the radius of the borehole. As the pad and transducers are moved along borehole, the transducers provide side-by-side paths at a fixed separation on the borehole wall which is independent of the borehole diameter. Each measurement transducer is employed to provide generally identical investigations representative of formation characteristics immediately adjacent the respective transducer and along the side-by-side paths. In this manner, the signals differ when formation characteristics change laterally between the transducers. The signals produced from the side-by-side path investigations are recorded in the manner which is adapted to comparing their likeness at a given depth to determine changes in lateral characteristics of the formation between the side-by-side paths.
In a further embodiment, first and second signals produced over a small depth interval are automatically compared to provide an indication of lateral characteristics between these signals. Comparisons are made with various depth displacements between the signals to determine the best likeness and corresponding depth displacement. A function of the best likeness and corresponding displacement is then recorded as an indication of a lateral characteristic of the earth formation between the side-by-side paths for the depth interval.
Signals produced in accordance with the invention from two closely spaced side-by-side transducers mounted on a common pad may be compared with far more certainty than signals obtained from separate pads with substantial and perhaps varying distances therebetween. The common pad embodiment employed by the invention assures the same operating environment for both transducers used to produce the compared signals. In contrast, it is well known that signals obtained from separate pads may vary, not of reason by changes in formation characteristics, but by reason of changes in the manner in which the different pads contact the borehole wall. Thus, a detailed comparison of two signals acquired from presumably identical measurement electrodes but located on separate pads introduces the uncertainty as to whether differences in the two signals correspond to lateral changes in characteristics of the formation or in differences related to the different pads and their position on the borehole wall. Comparison of signals obtained from side-by-side transducers on a common pad advantageously removes this uncertainty.
In the preferred embodiment the transducers take the form of substantially identical electrodes symmetrically mounted on a pad. The pad takes the form of a focussing device focussing each electrode in the same manner to assure substantially the same investigation of the formation immediately adjacent the electrode. The preferred investigation corresponds to a resistivity measurement of the type typically employed in resistivity dipmeter logging. However, in contrast with that practiced in dipmeter signal processing, the signals produced for comparison in accordance with the invention are obtained from side-by-side electrodes mounted in a fixed relation on the same pad whereas in dipmeter logging, the signals processed to obtain dips are obtained from different pads, which by nature of the type of linkage employed, have a variable distance therebetween, and by nature of the number of pads, are spaced circumferentially at 90.degree. or 120.degree. intervals about the borehole circumference. As previously explained, even when a given signal feature can be traced around the borehole from one signal to another, there is no assurance that the difference between the features on the different pad signal is representative of either lateral change in formation characteristics or representative of differences in pad to borehole wall contact.
The preferred comparison technique employed in accordance with the invention comprises successive correlations between the two signals which have a known and fixed distance between the signals sources with each correlation considering a different interval on at least one of the signals. Each such correlation provides a likeness factor. The best likeness factor and the corresponding displacement are determined. Functions of the best likeness and corresponding displacement are recorded as indications of lateral changes in characteristics of the earth's formation between the side-by-side paths for the compared signal interval.
Apparatus is disclosed for automatically and continuously comparing two signals produced as the pad bearing probe is moved along the borehole. The resulting best likeness factors and corresponding displacements may be continuously recorded versus borehole depth along with the orientation of the pad relative to magnetic North, for example.
In accordance with a multi-pad embodiment of the invention, pads containing pairs of side-by-side transducers may replace the pads normally employed for dipmeter apparatus. In this manner, the typical four-pad dipmeter may now produce eight dipmeter signals each corresponding to a different path on the borehole well. This doubling of the number of correlatable signals and the corresponding decrease in the distance between the paths for such signals provides a considerable advantage for dip determination.
For example, the best likeness and corresponding displacement determined between each pair of side-by-side transducer signals may be advantageously employed to determine the dip of the formation. Signals from a pad exhibiting poor likeness and therefore indicating changing lateral characteristics may be disqualified from the dip determination. If the likeness factor determined from side-by-side signals is good, both or either of these signals may be used for dip determination with assurance that the signals represent laterally correlatable formation characteristics. Further, the displacement corresponding to the best likeness between signals from one side-by-side transducer pad may be used to guide displacement determinations between signals from different pads to determine the dip of the formation, since laterally homogeneous features found between two side-by-side paths may be used to project the position of lateral features on adjacent paths. Still further, the displacements or lateral inclinations determined between best likeness features on side-by-side transducer signals obtained from several pads constructed in accordance with the invention may be combined to provide the dip of the laterally extending formation characteristics.
These and other advantages and objects of the invention can be understood from the following detailed description of method and apparatus for investigating lateral changes in characteristics of an earth formation described in conjunction with accompanying drawings wherein: